use v5.12;
my $check = "a";
$check++ for 1..3; # three letters
say $check;
$check++ for 1..24; # another 24 letters, but there's only 26 in the +alphabet,
say $check; # so what happens now we've run off the end?
# hmmm... that's handy.
# it takes us to the next part of your question...
my $input = $check;
my $output = substr($input, 1, 1); # skip one character, take one char+acter
say $output;

Very basic and better answered for yourself by reading perldoc perlintro (from your command line)... and -- as is often the case -- correctly answered in many different ways. Here is a (partial) answer for each question:

The first, obviously, is a simplied loop using the range operator (alternate syntax is shown by other replies above); the second takes advantage of the otherwise little-used chop. Note that it will "work" even if your charpair is NOT an ordered pair.

Please take these for examples, and set yourself the task of finding at least two other ways to answer each of your questions.

If you didn't program your executable by toggling in binary, it wasn't really programming!

Went to join the gridlock to see it
Held an eclipse party
Watched a live feed
I cn"t see tge kwubosd to amswr thus
I tried to see it, but 8000 miles of rock got in the way
What eclipse?
Wanted to see it, but they wouldn't reschedule it
Read the book instead